Refractory bodies containing boron nitride



in specific fields of use.

i purposes.

United States Patent REFRACTORY BODIES CONTAINING BORON. NITRIDE KennethM. Taylor, Lewiston, N.Y., assignor to The Carborundum Company, NiagaraFalls, N.Y., a corporation of Delaware No Drawing. Application March 12,1956 Serial No. 570,666

8 Claims. (Cl. 106-44) This invention relates to articles of manufactureand to compositions and methods for making them. More particularly, itrelates to bonded bodies or shapes composed essentially of boronnitride, with or without a silicon carbide filler, and a silicon carbidebond, and methods for making the same.

This application is a continuation-in-part application of my applicationSerial No. 288,552, filed May 17, 1952, now abandoned.

There is a constant search for new compositions or bodies which willpossess unexpected combinations of properties essential to or generallyfound to be desirable The boron nitride bodies of the present inventionin'which boron nitride, with or without a silicon carbide filler, isbonded by silicon carbide, possess certain combinations of propertiesand characteristics which render them of considerable value,

and they offer outstanding possibilities in a number of fields of use.It is, therefore, to be understood that the Silicon carbide bonded boronnitride bodies hereinafter more fully described are not to be consideredas restricted to any particular field of use. ing; characteristics asrefractory materials are particularly worthy of note and make themespecially suitable for a number of refractory purposes. The presentinvention will therefore be primarily described in respect to the use ofthe herein described products for refractory purposes, although notintended to be limited thereto.

Above all, a refractory body must possess refractoriness, that is, anability to stand up under exposure to high temperatures without unduechemical or physical change. Other desirable characteristics sought in arefractory body or shape include an ability to resist sudden changes intemperature Without cracking or other manifestations of body breakdown,a satisfactorily high mechanical strength at elevated temperatures aswell as at room temperature, chemical inertness and resistance .tovarious corrosive and erosive substances and conditions, a resistance tooxidation, and a density and hard- ..;-.ness dependent upon the use towhich the refractory body is to be put.

In order to obtain a high degree of perfection in respect of one or moreof the above properties peculiarly desirable for the specific refrectorypurpose in mind it has usually been found necessary to forego thebenefits of maximum performance in respect of certain other desirableproperties. Consequently, various refractory However, theiroutstandcompositions exceptionally suitable for one field of use It isanother object of the present invention to provide refractory bodies orshapes having a particular combination of refractory propertiesheretofore unavailable in refractory compositions.

It is another object to provide bonded boron nitride bodies in which theboron nitride is held together by means of a silicon carbide bond.

It is a further object to provide practical methods for making sucharticles.

In accordance with the present invention shapes or bodies composedessentially of boron nitride, with or without a silicon carbide filler,and a silicon carbide bond are formed by mixing the boron nitride, withor without a silicon carbide filler, with finely divided silicon metal,with or without the addition of a small amount of temporary binder orplasticizer to provide green molded strength, compressing a mass of thematerial or forming. an article of the desired shape by any of thewell-known methods of formation such as pressure molding, tamping,slip-casting, extrusion or the like, drying the formed article andfiring it in an atmosphere of carbon monoxide at a temperature and for aperiod of time sufficient to convert the silicon metal to siliconcarbide. If desired, the articles of the present invention after dryingcan be fired in a carbon monoxide atmosphere throughout the entirefiring schedule. The carbon monoxide atmosphere can be obtained by theintroduction of carbon monoxide as such from a suitable source of supplyor the carbon monoxide can be obtained by using oxygen or carbon dioxideand passing the gas through heated charcoal at a temperature to convertit substantially entirely to carbon monoxide as it is used. However, mypreferred practice is to raise the temperature of the article to thedesired firing temperature in an atmosphere ofhelium or other inert gasand after the firing temperature has been reached replacing the inertatmosphere by a carbon monoxide atmosphere for the duration of thefiring scheduleat the upper temperature ranges of the firing scheduleafter which the carbon monoxide is replaced again by an inert gas suchas helium and the temperature of the article lowered. This latterprocedure is preferable because it lessens the development and formationof free carbon. in the body of the article. t

Although I prefer to fire the herein described bodies of boron nitridein an. open controlled atmosphere of carbon monoxide as hereinabovedescribed, the present process can be modified in respect of the firingprocedure asfollows. The articles. to be fired can be fired in methaneor other carbonaceous atmosphere which will provide a source of carbonwithin the articles or they can be embedded in a bed of coke or othergranular carbon and tired at the same temperatures used in firing in anopen carbon monoxide atmosphere. The air, in passing through thesurrounding bed of coke or other carbon, has the oxygen content reducedto carbon monoxide by the time it reaches the embedded bodies, whichcarbon monoxide reacts with the silicon metal in the body to formsilicon carbide. It is noted that the nitrogen of the air likewisepasses through the embedding carbon and enters the bodies of the articlebeing fired, but my experience has shown that the reaction of thesilicon with the carbon monoxide appearsnto take preference over thereaction between the silicon and nitrogen at temperatures of 1200-1300C., so that the formation of silicon carbide predominates, although somelesser formation of silicon" nitride might take place.

In order to convert the silicon metal in substantial entirety to siliconcarbide the silicon metal should be in the neighborhood of 200 mesh(U;S. StandardSievej size or finer. The silicon carbide which is formedin situ from the reaction of the silicon metal with carbon monoxide isof the cubic crystalline, variety unidentified by X-ray diffractionanalyses and serves as an interstitial matrix bond to strongly unite theboron nitride and, when silicon carbide filler is included to unite thesilicon carbide also, to provide a body of good mechanical strength.

The amount of silicon carbide bond in the final article is not criticaland may be determined by the degree of mechanical strength of thefinished article required as well as the other properties sought in thefinal article.

As the amount of silicon carbide bond is increased the hardness andmechanical strength of the article is raised.

Highly satisfactory bodies have been obtained with the silicon carbidebonding matrix amounting to anywhere from 7% to 77% by weight of thebody of the article, which requires the use of to 70% by weight ofsilicon in the raw batch. Although the silicon carbide bond can be ashigh as 77% by weight of the article, i.e., 70% by weight of silicon inthe raw batch, it is preferable to have the amount of silicon carbidebond no higher than around 49% by weight of the article, i.e., no morethan 40% by satisfactorily used in carrying out the present inventiondiscloses, in addition to the silicon, the presence of the followingimpurities:

Percent Iron 0.87 Chromium 0.21 Aluminum 0.60 Calcium 0.54

The boron nitride used in carrying out the present invention may be acommercial grade of boron nitride material available on the market.However, I prefer to use a boron nitride material made in accordancewith the process described in my copending application Serial No.288,553, filed May 17, 1952. That method can be briefly described ascomprising forming a porous pelleted mixture of boric acid or boricoxide and tricalcium'phosphate and nitriding the pelleted mixture byheating it in a suitable furnace at around 900 C. in an atmosphere ofammonia for several hours whereby the boric oxide or acid is convertedto boron nitride. After the nitriding .step the resulting nitridedpellets are crushed and treated with dilute hydrochloric acid todissolve the tricalcium phosphate and other extraneous material. Theundissolved boron nitride after several washings with water is usuallytreated with hot 95% alcohol solution to further lower the content ofoxidic material and dried by allowing to stand overnight at roomtemperature followed by heating for 2 hours at 300 F. Analysis of theresulting boron nitride is as follows:

The 13.26% of extraneous matter in the above table of analysis of theboron nitride product has not been fully identified as to character butinsofar as it has been able to be determined it is considered to be forthe most part oxygen which is eitherphysically absorbed or united to theboron nitride in such a way that it is not alcoholsoluble as would bethe case if it were present in the form of boric oxide. Although thematerial before being hot pressed into a shaped body does not containany alcoholsoluble boric oxide, the shaped bodies resulting from hotpressing the material are found to contain a certain amount of freeboric oxide. It is therefore concluded that a certain amount of aphysically or chemically combined oxygen complex is contained in theoriginal material, although X-ray analyses reveal the presence only ofboron nitride. The analysis given above is therefore complete as far asit has been possible to positively identify the composition.

In order that the invention may be clearly understood, the followingexamples are submitted as illustrative of the compositions for andmanner of carrying out the present invention:

EXAMPLE I Small test bars 1 /2" x V2" x A to V2" in size, as well asnozzles 1%. inches in length and A3 inch in diameter, were made bymolding mixtures of boron nitrideand silicon metal, such as those setforth in Table I below, at room temperature and firing the molded shapesin an atmosphere of carbon monoxide for 2 hours at' approximately 1400C.

Two alternative procedures in firing were used. In one, a carbonmonoxide atmosphere was maintained throughout the entire heating up andcooling periods as well as during the period of sustained maximumtemperature. In the other, the carbon monoxide atmosphere was maintainedduring the holding period at sustained high temperature only, its. 1200"to 1400 C., and preferably above 1300" C., a hydrogen or heliumatmosphere being used during the heating up of the article to theholding temperature and again in the course of the cooling of thearticles to room temperature.

The products obtained by the two methods differed in some respects. Whencarbon monoxide is used throughout the entire firing schedule, theweight gain is usually as much as twice that theoretically required toconvert the silicon in the molded shape to silicon carbide, and theproduct is quite dark in color. When an inert atmosphere is used duringthe heating up and cooling periods, the product is lighter in color andthe weight gain is more nearly that calculated for the conversion of thesilicon to silicon carbide.

According to X-ray diffraction analyses of the resulting bodies thesilicon carbide derived from the silicon in the course of firing wasidentified as cubic silicon carbide. The dark color and the extra weightgain obtained when the article is fired with carbon monoxide atmospherethroughout the heating and cooling periods as well as the sustained hightemperature period of firing is considered to be due at least in part tothe deposition of free carbon within the article.

Table I below sets forth the composition and fabricating data and alsosome of the physical properties for a number of bar-shaped test piecesmade in accordance with the present invention. These test bars set forthin Table I were 1 /2" in length x /fi" wide and approximately .3 inchthick and were pressed at 30,000 pounds per square inch using fivepercent Carbowax No. 4000 as a temporary binder. According to the HandBook of Material Trade Names by Zimmerman and Lavine (published byIndustrial Research Service, Dover, New Hampshire, 1953), page 110,Carbowax is a group of nonvolatile, solid polyethylene glycols, solublein both water and aromatic hydrocarbons. They resemble natural waxes inappearance and texture, but are soluble in a much wider range ofsolvents. Their aqueous solutions possess binding properties. The samesource of authority states that Carbowax N0. 4000 is a hard, waxy solidhaving specific gravity of 1.2, freezing range of 5 0-55 C., a flashpoint greater than 475 F., and a Saybolt viscosity of 500700 seconds at210 F. The boron nitride and silicon metal in finely divided form wereintimatelymixed. To the mixture of boron nitride and silicon metal wasadded by weight of the total mass of Carbowax" as a temporary binder andthe resulting mixture molded to the d esired shape. The temporary binderwas removed by heating the molded bodies for a few hours at BOO-400 C.The resulting shapes were then fired at 1400 C. for a sufiicient periodof time to convert the silicon in the body to silicon carbide. Theatmosphere during the heating up and cooling phases of the firingschedule was either carbon monoxide or hydrogen as prescribed in TableI.

Table I 6 No.1 had a density of1.68 grams'per cubic centimeter; No. 21had: a density of 1.54 grams per cubic centimeter; and t No. 3 -hadadensity'of 1 .68 gramsper. cubic centimeter.

EXAMPLE 11 Rocket nozzles 1%" inches in length and V8 inch in diameterwere made in accordance with a modified form of the present inventionwherein a silicon carbide filler BAR SHAPED OOMPAO'TS OF MIXTURES OFBORON NITRIDE AND SILICON FIRED IN CARBON MONOXIDE FOR TWO HOURS AT 1400C.

Sandblast penetration,

inches 1 Standard penetration on plate glass when subject to the samepenetration test is .010

of an inch.

The effect of the atmosphere used during the heating was included in thecomposition from which the article up and cooling periods can be readilyseen by a comparison of the difierence in weight gains between theexperimental bars 3 and 5 and also the comparison between the weightgains of bars 6 and 7 of the above table. It is noted that when carbonmonoxide is used throughout the entire firing schedule the bars gainmore in weightduring firing, are of higher apparent density and aresomewhat harder. The reasons for these differences are not fullyunderstood, although, as previously suggested, it is believed that freecarbon is formed in those bodies fired in carbon monoxide for the entireschedule.

The boron nitride used in making experimental bars 6, 7 and 8 wasprefired. in an ammonia atmosphere for 8 to 14 hours at 1400 C. beforebeing mixed with the silicon. The resulting fired compacts are softerthan those bars in which untreated boron nitride was used. It was alsonoted that compacts or bars embodying prefired boron nitride expandedmore in firing than bars made from untreated boron nitride.

I have, further found that when the boron nitride is prefired prior toits mixture with silicon and further firing i that the resulting articlehas a somewhat higher resistance to heat shock.

Nozzles 1% inches in length and inch in diameter weremade from mixturesof boron nitride and silicon as follows. The composition of these andthe atmosphere maintained in the furnace during the heating up andcooling periods were as follows:

The nozzles were formed, previous to firing, by pressing at 8,000 to10,000 pounds per square inch with fifteen percent of Carbowax No. 4000as a temporary binder.

The approximate apparent densities of the fired nozzles were as follows:

was made, in accordance with the following mixture:

Percent by weight Silicon carbide (400 grit. mesh size) 40 Silicon (10microns andfin'er) 40 Boron nitride 2 0 The nozzles were molded at 8,000to 10,000 pounds per square inch pressure with fifteen percent ofCarbowax No. 4000 as a temporary binder. The pieces were'fired for twohours at 1400 C. in an atmosphere of carbon monoxide at the sustainedhigh temperature, a helium atmosphere being used during the heating upperiod and also during the cooling of the articles after the two hourfiring period. The fired nozzles had an apparent density of 1.95 gramsper cubic centimeter. They were strong and hard and a uniform darkgrayish blaclr color in appearance.

While I have described in the above examples the making of variousmolded shapes in which the article is molded and fired in the exactshape or form in which it is intended for use, the present invention isnot intended to be so restricted. Another way of making and usingsilicon carbide bonded boron nitride bodies of the present invention isto mold the raw batch of material into briquettes or other shapes orotherwise compress a mass of the material having a composition in thedesired proportions, after which the resulting briquettes or compressedbodies are fired in the manner already described. After removal from thefurnace, they are crushed to granular form of the desired gr-it size.The resulting granular material can then be used in loose granular formas a high temperatureinsulation material, as, for example, insulationaround jet engines and rocket combustion chambers, or as. a layer ofinsulation around industrial furnace chambers. It may also be used as aloose filtering media or as a catalyst or catalyst carrier material- Thegranular materialcan also be bonded by means of sintered metals,vitreous or ceramic bonds. or other bonding materials to form articlessuitable for many of: the industrial uses set forth elsewhere herein.

Likewise, articles or bodies can be made in accordance with thepresentinvention in which pore-forming materials are incorporated in the rawbatch from which the body is made for the purpose of providing a greaterdegree of porosity in the final body. Pore-forming material such ascarbon and the like, which requires oxidation for removal from a bodywould require a preliminary burning out of the pore-forming material atlower temperatures. Therefore, the pore-forming material preferablyshould be a material which is removed by volatilization during thedrying and/or firing operation such as powdered or granular naphthalene,various organic resinous materials such as phenolic resins and the likeor one which provides pores by reason of the generation of a gas. Theresulting bodies, which have greater porosity than those made with nopore formers, are particularly useful in the fabrication of porousfiltering media, catalysts and catalyst carriers, insulation bodies andthe like, whether in crushed granular form or in the form of moldedshapes of predetermined contour.

It is to be understood that the products of the present invention in itsvarious modifications are not limited to any specific field or fields ofuse such as might be defined by the specific examples previously setforth. The products can be made in any desired shape as Well as providedin granular or aggregate form. They are, therefore, not only suited formany of the uses for which industrial refractories are required,including bricks, blocks, setter tile, mufiies, kiln furniture andspecial shapes for application in and around furnaces and other hightemperature equipment, but they are also well suited for many specialtyhigh temperature applications, such as jet engine combustion chambers,linings for exhaust nozzles, rocket combustion chambers and exhaustnozzles, turbine blades, stator blades, lens fusion blocks, spark plugbodies, and the like. The bodies are also suitable for making cruciblesand other laboratory ware or industrial structural articles or parts forthe handling of corrosive chemicals such as molten cryolite or otherfused halides. They are also suitable for the fabrication of laboratoryware, including combustion boats, crucibles, burner holders, and othershapes. The bodies of the present invention, particularly when modifiedby the use of pore formers in the raw batch from which the bodies aremade, are also highly useful as diifustion filtering media, such asdiffusion tubes and plates, filtering tubes, plates and shapes, or ascatalysts or catalyst carriers, radomes for guided missiles, etc.Materials and articles of the present invention can also be used formaking abrasive articles such as grinding wheels, sharpening stones,razor bones, and other grinding and polishing shapes and materials. Thepresent bodies offer possible applications in the electrical and radioindustry including supports in electric light bulbs, radio tubes, X-raytubes and radar equipment, resistors and grid leaks.

Having described the present invention it is desired to claim:

1. A method of making bonded boron nitride articles of manufacture whichcomprises forming an article of the desired shape from a mixtureconsisting essentially of boron nitride particles and to 70% by weightof finely divided silicon, and heating said article in an atmosphere ofcarbon monoxide at'a temperature sufficient to react the carbon monoxideof the atmosphere with the silcon to form silicon carbide and therebybond the boron nitride particles together.

2. A method of making bonded boron nitride articles of manufacture whichcomprises forming an'article of the desired shape from a mixtureconsisting essentially of boron nitride particles, silicon carbideparticles and 5% to 70% by weight of finely divided silicon, and heatingsaid article in an atmospherev of carbon monoxide at a temperaturesufficient to react the carbon monoxide of the atmosphere with thesilicon and form silicon carbide and thereby bond the boron nitrideparticles together.

3. A method of making bonded boron nitride articles of manufacture whichcomprises forming an article of the desired shape from a mixtureconsisting essentially of boron nitride particles and 5% to 70% byweight of finely divided silicon, and heating said article in anatmosphere of carbon'monoxide at a temperature of 1200. to 1400 C. 1

4. A method of making bonded boron nitride articles of manufacture whichcomprises forming an article of the desired shape from a mixtureconsisting essentially of boron nitride particles and 5% to 70% byweight of finely divided silicon, heating said article up to atemperature of around 1300" C. in an inert atmosphere, replacing theinert atmosphere with a carbon monoxide atmosphere, holding the articleabove 1300 C. in said atmosphere of carbon monoxide for a period of timeto convert the silicon in the article to silicon carbide, replacing thecarbon monoxide atmosphere with an inert atmosphere and lowering thetemperature of the article.

5. A method of making bonded boron nitride articles of manufacture whichcomprises forming an article of the desired shape from a mixtureconsisting essentially of boron nitride particles and 5% to 70% byweight of finely divided silicon, heating said article up to atemperature of around 1300 C. in an atmosphere of helium, replacing thehelium by carbon monoxide and holding the article above 1300 C. in anatmosphere of carbon monoxide for a period of time to convert thesilicon in the article to silicon carbide, replacing the carbon monoxideatmosphere with an atmosphere of helium and lowering the temperature ofthe article.

6. A method of making bonded boron nitride articles of manufacture whichcomprises forming an article of the desired shape from a mixtureconsisting essentially of boron nitride particles and 5% to 70% byweight of finely divided silicon, and disposing said article in anenvironment providing carbon to react With said silicon, and heatingsaid article to a temperature sufficient to react the carbon with thesilicon to form silicon carbide thereby to bond the boron nitrideparticles together.

7. A method of making bonded boron nitride articles of manufacture whichcomprises forming an article of the desired shape from a mixtureconsisting essentially of boron nitride particles and 5% to 70% byweight of finely divided silicon, embedding said article in a mass ofgranular carbon, and firing the article while so embedde at atemperature of 1200 to 1400 C.

8. A method of making bonded boron nitride articles of manufacture whichcomprises forming an article of the desired shape from a mixtureconsisting essentially of boron nitride particles, granular siliconcarbide and 5% to 70% by weight of finely divided silicon, embeddingsaid article in a mass of granular carbon, and firing the article whileso embedded at a temperature of 1200 to 1400 C.

References Cited in the file of this patent UNITED STATES PATENTS2,108,794 Boyer et al Feb. 22, 1938 2,109,246 Boyer et a1. Feb. 22, 19382,119,489 Beer May 31, 1938 2,431,327 Geiger Nov. 25, 1947 2,609,318Swentzel Sept. 2, 1952 2,636,825 Nicholson Apr. 28, 1953 2,636,826Nicholson Apr. 28, 1953 FOREIGN PATENTS 478,016 Great Britain Jan. 11,1938

1. A METHOD OF MAKING BONDED BORON NITRIDE ARTICLES OF MANUFACTURE WHICHCOMPRISES FORMING AN ARTICLE OF THE DESIRED SHAPE FROM A MIXTURECONSISTING ESSENTIALLY OF BORON NITRIDE PARTICLES AND 5% TO 7% BY WEIGHTOF FINELY DIVIDED SILICON, AND HEATING SAID ARTICLE IN AN ATMOSPHERE OFCARBON MONOXIDE AT A TEMPERATURE SUFFICIENT TO REACT THE CARBON MONOXIDEOF THE ATMOSPHERE WITH THE SILCON TO FORM SILICON CARBIDE AND THEREBYBOND THE BORON NITRIDE PARTICLES TOGETHER.